This invention relates to test stands for measuring and adjusting fluid flow devices. In particular, this invention relates to such test stands which use calibrated test nozzles for determining the fluid flow characteristics of a specimen, such as a carburetor.
Prior U.S. Pat. Nos. 3,520,312 and 3,528,080 disclose a method and apparatus for testing the flow characteristics of a test specimen, such as a carburetor. In accordance with the disclosure of those patents, air is drawn through the test specimen by an exhaust pump from a temperature and pressure controlled enclosure. The test stand is provided with a number of sonic outlet nozzles. While the drawings of the prior patent show two outlet nozzles, the specification indicates that a larger number of such nozzles are required in order to adequately test the operation of various carburetors under various operating conditions. Extensive testing of air and fuel flow through a carburetor is required to assure that the carburetor will function properly and provide a suitable fuel-air mixture, so that the associated engine will operate in compliance with emission control standards. A similar test stand can be used for testing fluid flow characteristics of specimens other than carburetors, such as thermostatically controlled venting devices.
A larger number of sonic outlet nozzles are required for adequate testing of various specimens because the outlet pressure of the device must be matched to the inlet pressure of the test nozzle with the identical volume of fluid flow through the specimen and the test nozzle. Accordingly, prior art devices made use of different aperture size nozzles, used alone or in combination, to adjust the fluid flow volume through the test stand and thereby match the pressure and flow volume characteristics of the test nozzles and the specimen.
In accordance with the prior art, the test specimen is supplied with air, or other fluid, at a selected temperature and pressure. The outlet of the test specimen is connected to an intermediate chamber which has an outlet through a test nozzle. The test nozzle outlet opens into a chamber which is evacuated to less than half the pressure in the intermediate chamber so that the flow through the test nozzle is at sonic velocity, and therefore dependent only on the pressure in the intermediate chamber. Since the specimen and test nozzle are in series flow configuration, the flow volume through the specimen is equal to the flow volume through the test nozzle. This flow volume can be determined directly from the intermediate chamber pressure, since the outlet test nozzle is in a sonic or critical flow condition. A proper measurement of the specimen flow characteristics is made when the intermediate chamber pressure, which is below the specimen inlet pressure, causes an equal flow volume through the test specimen and the test nozzle.
The pressure versus flow volume relations illustrated in FIG. 1 include that of a carburetor, curve I, and that of a selected outlet test nozzle, curve II. For the characteristics illustrated, it is evident that the test nozzle does not have a pressure and flow volume condition which corresponds to one of the pressure and flow volume conditions of the particular specimen. Thus, at a constant flow condition d1 a pressure p1' is required at the specimen outlet and a lower pressure p1 is required to maintain the flow through the outlet test nozzle. If a selected pressure p1' exists in the intermediate chamber between the test specimen and the outlet nozzle, the flow rates through the two devices will correspond to d1 and d1', which are unequal. It is therefore evident that the particular device represented by curve I cannot be properly tested using the outlet test nozzle represented by curve II. For this reason, the prior art device required the use of many outlet test nozzles, each with a different opening, and each having a different slope of pressure versus flow volume.
It is therefore an object of the present invention to provide a new and improved flow test stand wherein a single outlet test nozzle may be used for testing specimens having a variety of pressure and flow volume characteristics.